Gas sensors for sensing carbon monoxide (CO) and other toxic gases are used in a wide variety of applications, from commercial and residential markets to automobiles and recreational vehicles to scientific instrumentation. Several technologies are currently known for measuring carbon monoxide.
One type of toxic gas sensor is sometimes referred to as a semiconductor gas sensor. Typically such sensors use a ceramic semiconductor, such as tin oxide, in combination with a catalyst such as palladium or platinum for catalytic combustion of the toxic gas. Commercial examples include Figaro Model No. TGS-822 available from Figaro, Inc. (Japan). In the presence of a catalyst and heat, a toxic gas such as carbon monoxide undergoes a catalyzed reaction at the surface of the semiconductor, changing the conductivity of the sensor. Conductivity of the sensor rises as the concentration of toxic gas rises, and this change can be measured by detection circuitry.
Such semiconductor based sensors have several problems and limitations. Semiconductor based sensors operate only at high temperatures, typically in the range of 200.degree. C. to 500.degree. C. and therefore require a heating source to create a detectable electrical signal. Further, sensors of this type are sensitive to changes in humidity. Also, other oxidizable or reducible gases will readily react with the sensor in the normal, high temperature operating condition, increasing the chances of generating a false positive signal. Moreover, semiconductor based toxic gas sensors have inherent limitations in their sensitivity and accuracy and are expensive to manufacture.
Another type of toxic gas sensor is a gel type sensor, also referred to as a biomimetic type toxic gas sensor. In this type of sensor organo-metallic materials are used in a gel with a catalyst. Exposure to carbon monoxide changes the color of the gel. An optical scanner detects the change in the color of the gel. Such sensors, however, are slow to respond to changes in carbon monoxide concentration, never completely recover their original color after exposure to carbon monoxide and, to a greater degree than the semiconductor based sensors, have inherent limitations in accuracy.
Another type of toxic gas sensor is an electrochemical sensor, based upon use of a liquid acid electrolyte. Carbon monoxide is exposed to a cathode. A liquid acid electrolyte connected to the electrode acts as a protonic conductor, carrying protons between a cathode and an anode. An electric signal generated by the reaction is proportional to the concentration of carbon monoxide sensed at the cathode. Unfortunately, carbon monoxide sensors in accordance with this type of technology suffer from accuracy problems due at least in part to corrosion by the acid, by the liquid drying over time, and the inherent limitations in the signal generated by the liquid acid. Furthermore, the acidity of such sensors changes with time. Change in the acidity of these kinds of liquid electrolytes causes these kinds of sensors to quickly lose accuracy. In addition to these problems, to measure the concentration level of the toxic gas which is desired to be detected, an electric potential (typically provided by a DC power source) must be applied across the sensor. Constant application of a DC power source acts to polarize the sensor, so that the electric signal generated drifts over time with the changes in this polarization voltage.
Recently, more robust, low cost and accurate electrochemical toxic gas sensors have been developed, based on the use of solid protonic conductive polymer membranes. Examples of protonic conductive membrane based toxic gas sensors include U.S. Pat. No. 5,573,648 to Shen et al and U.S. Pat. No. 5,650,054 to Shen et al. When the sensor is exposed to a toxic gas such as carbon monoxide, an electric signal is generated. The greater the carbon monoxide level, the greater the electric signal. More specifically, carbon monoxide reacts with moisture in the air at room temperature at a sensing electrode to produce carbon dioxide, protons and electrons in a reaction as follows. EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +2H.sup.+ +2e.sup.-
The protons (hydrogen ions) migrate across a protonic conductive electrolyte membrane to a counting electrode where they react with oxygen in a reaction as follows. EQU 2H.sup.+ +2e.sup.- +1/2O.sub.2.fwdarw.H.sub.2 O
Sensors of this type are advantageous in that, in addition to substantially improved accuracy, they can function with no need for an external power source to force the reaction. This reduces costs of the sensor and reduces power consumption, a feature which is advantageous for battery operated devices.
It is desirable to have the electrodes formed thin to minimize internal electronic resistance. In the '054 patent to Shen et al, one of the electrodes 14 has a portion which necessarily extends down into contact with a current collecting bottom washer around a nonconductive membrane in order to complete the electrical connection through the sensor. This is inconsistent with the objective of making the electrodes as thin as possible.
It would be highly desirable to provide an improved toxic gas sensor which would have an enhanced electric signal generated in the presence of toxic gas, especially carbon monoxide, to facilitate improved measurement, accuracy and reduced response time by a toxic gas detector employing such a sensor. It would also be highly desirable to provide an electrochemical toxic gas sensor which protects its electrodes from exposure to liquid water. Further, it would be highly desirable to provide an improved toxic gas sensor which is operable under a wide range of temperatures. It would also be highly desirable to provide an electrochemical and toxic gas sensor which has self-diagnostic features, and to provide a toxic gas sensor having self-calibration features.
In view of the foregoing, it is an object of the present invention to provide a gas sensor for sensing carbon monoxide and other toxic gases having an electrical signal generated in response to sensing a toxic gas and to provide protection for its electrodes from exposure to liquid water. It is an additional object of the present invention to provide a carbon monoxide and toxic gas sensor for use in a gas detector having fast response times. It is an additional object of the present invention to provide a gas sensor which operates properly even under very low temperatures. It is still another object of preferred embodiments of the present invention to provide an electrochemical toxic gas sensor having a self-diagnostic feature to indicate when the sensor is not functioning normally, and/or a sensor having a self-calibration feature to indicate when the sensor is accurately measuring the amount of toxic gas present in the sensed environment. It is an additional object of preferred embodiments of the present invention to provide a carbon monoxide and toxic gas sensor that is of low cost, compact size, easy to manufacture and which is highly reliable in operation.